There exist a variety of techniques for detecting properties of materials. However, generally these techniques are limited to detection at discrete points within a material and require timely and sometimes destructive evaluation of a material. Such analysis is further compounded when materials contain anisotropic properties. Therefore, there exists a need to evaluate the characteristics and properties of materials such as solids, liquids and gases to determine directional variations in such properties. For example, there exists a need to determine anisotropic properties within solid material plates or sheets such as in sheet metal and paper products.
One technique involves the monitoring of traveling elastic waves as they pass through a material. Such waves will vary in speed in proportion to changes in material properties, giving an indication of such properties. For example, pulsed holographic recording has been used to a limited extent to image synchronized traveling elastic wave motion. Another technique is Schlieren photography which has been used to image synchronized traveling elastic wave motion in optically transparent and diffracting media. However, such technique is limited to detecting traveling wave motion within only optically transparent and diffracting media. Yet another technique involves electronic speckle pattern interferometry (ESPI) which is used to image large motions. Such technique is commercially available. However, the image processing is relatively complex since it requires external post processing for extraction of the measurement information.
One technique for implementing ultrasonic non-destructive evaluation of materials involves testing a plate or sheet of material with an emitter and a detector. The emitter, a contact piezoelectric device, is positioned relative to the detector to measure travel velocity, or elastic wave speed, along a specific direction in a material extending between the emitter and detector. The emitter is positioned at a central location against the material, and the detector is placed at one of a plurality of discrete locations along a circle extending about the emitter. Measurements are taken at each discrete location by moving the detector to each location, and travel time is calculated to determine the velocity in each direction. Accordingly, velocity differences in specific directions can be correlated with anisotropic material characteristics. For example, metal plates have been investigated in a water bath using an acoustic generator to determine travel velocity in a given direction at a point on the plate. An air coupled transducer or laser ultrasonics device enables non-contacting determination of anisotropy in materials. However, such point measurement techniques are slow and difficult to automate because the detector must be positioned or moved to each of a number of discrete locations about the emitter in order to detect travel speed along all directions in the material under test.
Another technique for implementing non-destructive evaluation of materials involves the use of a coherent laser to illuminate an object and form an interference pattern related to changes on the surface of the object. Variations include the use of holography, Electronic Speckle pattern Interferometry (ESPI) and Shearography. The interference pattern is then recorded with a camera such as a charge coupled device (CCD) camera. The resulting image is then recorded and processed to produce an image of the surface displacement. However, the minimum detectable displacements typically range from 10-100 nanometers. Furthermore, such techniques require the use of external image processing to produce a usable output.
While the above-described techniques have provided some degree of success, there exist several shortcomings needing resolution. For example, there exists a need to provide imaging of synchronous traveling wave motion at arbitrary frequencies within solids, gases or liquids such that material properties can be determined. There also exists a need to provide rapid full-field imaging of a traveling wave displacement amplitude simultaneous for all points extending over a material surface. Finally, there is a need for imaging travelling wave motion occurring at small ultrasonic displacements (e.g., less than one (1) nanometer in amplitude).
Therefore, it is desirable to provide an apparatus and method for imaging traveling wave motion within materials. It is furthermore desirable to extend such imaging in order to determine material properties that relate to the velocity that such waves travel through a material. Furthermore, there is a need to provide for such apparatus and method with a simplified design and implementation that enables quick, or rapid full-field view imaging, is relatively low-cost, and enables non-destructive imaging and material testing with high sensitivity.
One object of the present invention is to provide a vastly improved traveling wave imaging apparatus and method particularly suited for use with diffusely reflecting surfaces and having a greatly enhanced sensitivity, linear output for small vibration amplitudes (proportional to Bessel function of order one), while simultaneously providing a rapid full-field image of a traveling wave propagating over the surface of the specimen while enabling surface imaging and material property characterization.